Single-crystal apatite nanowires sheathed in graphitic shells and synthesis method thereof

ABSTRACT

Heterogeneous nanowires having a core-shell structure consisting of single-crystal apatite as the core and graphitic layers as the shell and a synthesis method thereof are provided. More specifically, provided is a method capable of producing large amounts of heterogeneous nanowires, composed of graphitic shells and apatite cores, in a reproducible manner, by preparing a substrate including an element corresponding to X of X 6 (YO 4 ) 3 Z which is a chemical formula for apatite, adding to the substrate a gaseous source containing an element corresponding to Y of the chemical formula, adding thereto a gaseous carbon source, and allowing these reactants to react under optimized synthesis conditions using chemical vapor deposition (CVD), and to a method capable of freely controlling the structure and size of the heterogeneous nanowires and also to heterogeneous nanowires synthesized thereby.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to heterogeneous nanowires having acore-shell structure consisting of single-crystal apatite as the coreand graphitic layers as the shell and to a synthesis method thereof.More specifically, the present invention relates to a method capable ofproducing large amounts of heterogeneous nanowires, composed ofgraphitic shells and apatite cores, in a reproducible manner, bypreparing a substrate including an element corresponding to X ofX₅(YO₄)₃Z which is a chemical formula for apatite, adding to thesubstrate a gaseous source containing an element corresponding to Y ofthe chemical formula, adding thereto a gaseous carbon source, andallowing these reactants to react under optimized synthesis conditionsusing chemical vapor deposition (CVD), and to a method capable of freelycontrolling the structure and size of the heterogeneous nanowires andalso to heterogeneous nanowires synthesized thereby. The examples of thepresent invention show the results of using calcium or strontium as anelement corresponding to X of the chemical formula, the results of usingphosphine containing phosphorus for the formation of Y of the chemicalformula, and the results of using acetylene (C₂H₂), methane (CH₄),ethylene (C₂H₄) or propane (C₃H₈) as a gaseous carbon source for theformation of graphitic layers.

2. Description of the Prior Art

Crystalline carbon materials such as graphene are known as advancedmaterials having very excellent electrical, mechanical, chemical andphysical properties and have recently been used in a wide range ofapplications, including electrode materials, reinforced compositematerials, and optical materials.

Carbon nanowires are advanced materials having a shell structureconsisting of graphene rolled up into a cylinder shape having a diameterof nanometer order and can be broadly classified into carbon nanotubes,carbon nanofibers, and carbon nanocables. Carbon nanocables generallyrefer to structures in which materials having the shape of rods or wiresare included in the hollow spaces of graphitic shells, unlike carbonnanotubes or carbon nanowires.

When the material included in such carbon nanocable structures to formthe core is disadvantageous that it is sensitive to externalenvironmental factors (for example, it is easily to be oxidized, or isadversely affected by acid or is sensitive to water), or the mechanicaland physical properties are inherently weak or the electrical propertiesthereof are not excellent, the graphitic shells surrounding the surfaceof the core materials can overcome such disadvantages, and thus arehighly beneficial.

In addition to the fact that the inherent properties of the corematerial can be maintained by the graphitic shells, the properties ofthe core material which is very excellent in one of the electrical,mechanical, chemical and physical properties can be additionallyimproved by the graphitic shells.

Meanwhile, carbon nanocable structures can be formed by various methods.The most general method is an in-situ formation method that is based onchemical vapor deposition (CVD) or arc discharge. In this method, coreswhich can be present in carbon nanotubes are made mainly of transitionmetals having an excellent catalytic activity of forming carbonnanotubes. In recent years, nanosized metal oxides have been reported tobe able to form such graphitic shells. The vapor-liquid-solid (VLS)growth mechanism is primarily responsible for the synthesis of carbonnanocables by the above method.

Another method is a method of filling a liquid or gaseous material intoprepared carbon nanotube structures using capillary action, awet-chemical method, and a nano-filling reaction. In this method, massproduction is not easier than in the in-situ formation method, but thereis an advantage in that various materials can be used as the corematerial.

Meanwhile, it has not yet been reported that bio-minerals such asapatite can be formed directly into carbon nanotube structures whichcomprise, for example, graphitic shells. Calcium phosphate compounds aretypical minerals and can typically be developed to the chemicalstructure of apatite, which is generally represented by X₈(YO₄)₃(Z),wherein X may represent Ca, K, Na, Sr, Ba, Mg, Pb, Cb or Zn, Y mayrepresent P, As, V or S, and Z may represent OH⁻, F⁻, CO₃ ⁻ or Cl⁻.Compounds represented by the chemical formula have various propertiesand structures depending the components and composition ratios of X, Yand Z.

Particularly, compounds in which X is Ca and Y is P are calciumphosphate compounds which have various properties and structuresdepending on whether Z represents OH⁻, F⁻, O⁻, CO₃ ⁻ or Cl⁻. Specificexamples of the calcium phosphate compounds include hydroxyapatite:HA(Ca/p=1.67)-Ca₅(PO₄)₃(OH); fluoroapatite: (Ca/p=1.67)-Ca₅(PO₄)₃(F);carbonated apatite: (Ca/p=1.67)-Ca_(n)(PO₄)₈(CO₃)(OH), oxyapatite:OA(Ca/p=1.67)-Ca₁₀(PO₄)₆O; octacalcium phosphate:OCP(Ca/p=1.33)-Ca₈H₂(PO₄)₆5(H₂O); tricalcium phosphate:OCP(Ca/p=1.5)-Ca₃(PO₄)₂; tetracalcium phosphate:OCP(Ca/p=2.0)-Ca₄(PO₄)₂O; brushite: (Ca/p=1.0)-CaH(PO₄)2(H₂O); andmonetite: (Ca/p=1.0)-CaH(PO₄)).

These compounds generally have very excellent biocompatibility and areused mainly in the biotechnology field related to the production ofartificial teeth and bones, but are known to have low mechanicalstrength and insufficient electrical and chemical properties.

In addition, the synthesis of calcium phosphate compounds is generallycarried out in a moisture- or oxygen-rich atmosphere because of theirstructural characteristics. Such conditions for the synthesis of calciumphosphate compounds are significantly inconsistent with conditions forthe production of graphitic structure, and thus two kinds of materials(graphitic shells and calcium phosphate compounds) were difficult tosynthesize simultaneously under the same conditions. Accordingly, thereis a need to form composites of calcium phosphate compounds andgraphitic nanostructures, thereby improving the mechanical and physicalproperties of the calcium phosphate compounds.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide novel heterogeneousnanowires, which have a core-shell structure consisting of apatite asthe core and graphitic layers as the shell, and thus show very excellentelectrical, mechanical, chemical, physical and biocompatible properties,and a synthesis method thereof.

Another object of the present invention is to provide heterogeneousnanowires and a synthesis method thereof, in which an apatite core and agraphitic shell can be produced simultaneously by a single process andthe shapes thereof can be freely controlled.

Still another object of the present invention is to provideheterogeneous nanowires which can be applied in all the technicalfields, including the energy field and the nano/bio-technology field, inwhich apatite and graphitic shells are used, and a synthesis methodthereof.

To achieve the above objects, the present invention provides a methodfor synthesizing single-crystal apatite nanowires sheathed in graphiticshells, the method comprising the steps of: i) introducing into areactor either a material containing an element corresponding to X ofX₅(YO₄)₃(Z), which is a chemical formula for apatite, or a substratecontaining the material; ii) maintaining the inside of the reactor in avacuum and supplying a carrier gas to the reactor; iii) increasing thetemperature of the reactor to synthesis temperature; iv) supplyingreactant gases comprising carbon and phosphorus sources to the reactorand allowing the reactant gases to react with the material or substrateintroduced into the reactor in step i); and v) cooling the reactor toroom temperature in a carrier gas atmosphere.

In the method of the present invention, the element corresponding to Xin the chemical formula may be Ca, K, Na, Sr, Ba, Mg, Pb, Cb or Zn. Forexample, if the element is calcium, the material containing the elementis preferably a material which contains calcium or calcium oxide or iscapable of inducing calcium or calcium oxide. Examples of a biomaterialcontaining calcium include henequen and kenaf, which are woodybiomasses, red algae and brown seaweed.

The shape of the substrate may be selected from mesh, foams, spheres,fibers, tubes, plates, thin films, powders, and nanoparticles. In oneembodiment of the present invention, the substrate may be glass fiber orglass powder.

In addition, the substrate is preferably mounted in the reactor using anassistant material made of alumina or quartz which is stable to thereactant gases at a temperature ranging from room temperature to 1000°C. Furthermore, before the reactant gases are supplied, the internalpressure of the reactor is reduced to a vacuum of 1×10⁻³ Torr by, meansof a vacuum pump in order to remove the remaining gas from the reactor,and then the carrier gas is supplied to the reactor maintained in avacuum.

Moreover, the carrier gas includes any one of argon, helium andnitrogen. Before the reactant gases are supplied, the temperature of thereactor is preferably controlled to the synthesis temperature rangingfrom 500° C. to 1000° C.

Also, the carbon source-containing reactant gas that is supplied to thetemperature-controlled reactor preferably includes hydrocarbon gas suchas acetylene, ethylene ethane, propane or methane, and the phosphorussource-containing reactant gas preferably includes phosphine gas.

Herein, the phosphine gas is one example of a reactant gas that containsphosphorus (P) among P, As, V and S, which are elements corresponding toY in the apatite structure.

Also, the carbon source-containing reactant gas is supplied in order toform graphitic shells.

The reactant gases can react with each other during the synthesisprocess to form new gaseous carbon-phosphorus organic compounds. If thecarbon-phosphorus organic compounds are prepared in liquid form, theyare preferably evaporated by heating or atomized by ultrasonicatomization, before they are supplied to the reactor.

If the introduced substrate is a calcium-containing substrate, amorphouscalcium phosphate nanoparticles start to be formed on the surface of thesubstrate as phosphine is supplied. In this process, phosphorusmolecules thermally decomposed from phosphine at synthesis temperaturereact with the oxygen of the substrate to form phosphates which thenreact with calcium, thereby forming amorphous calcium phosphatenanoparticles. In addition, because the gaseous carbon-phosphate organiccompounds resulting from the reaction between the carbon and thephosphorus source-containing reactant gas can induce the strong bondingbetween the calcium and the phosphate, they can also promote theformation of amorphous calcium phosphate nanoparticles. Then, theamorphous calcium phosphate nanoparticles undergo a nucleation andcrystallization process, and the gaseous carbon-phosphorus organiccompounds also play a role in promoting this nucleation andcrystallization process and function to induce the apatite crystal to beoriented in one plane that is the (001) plane. When the phosphorus andcalcium-containing gaseous species are continuously supplied,one-dimensional apatite nanowires oriented in the (001) plane grow,while graphitic shells are formed around the surface of the grownapatite nanowires under supply of the carbon source-containing reactantgas. The graphitic shells formed around the apatite nanowires functionto block the phosphorus and calcium-containing gaseous species frombeing supplied to the radial surface of the nanowires, therebypreventing the lateral growth of the nanowires. In addition, the formedgraphitic shells function to promote the growth of the nanowire in theaxial direction. As a result, the inventive apatite nanowires sheathedin graphitic shells can be formed by a CVD-based vapor-solid (VS) growthmechanism. In this formation process, the gaseous carbon-phosphorusorganic compounds can play a very important role in the formation andoriented growth of apatite crystals.

In addition, the time of the reaction is preferably controlled withinthe range of 30 seconds to 2 hours.

In another aspect, the present invention provides single-crystal apatitenanowires sheathed in graphitic shells, synthesized by theabove-described method. In a preferred embodiment, apatite comprises99-100% of the hollow volume of the graphitic shells, the nanowires havea diameter of 5 nm to 20 nm and a length of 100 nm to 5 μm, and thegraphitic shells have a thickness of 0.34 to 2 nm. The heterogeneousnanowires may be used as biomaterials, nanomaterials, or nano/biocomposite materials.

In still another aspect, the present invention provides a method forsynthesizing heterogeneous nanowires, which are composed of graphiticshells and apatite cores and have a thickness which changes in the axialdirection thereof, the method comprising the steps of: i) introducinginto a reactor either a material containing an element corresponding toX of X₅(YO₄)₃(Z), which is a chemical formula for apatite, or asubstrate containing the material; ii) maintaining the inside of thereactor in a vacuum and supplying a carrier gas to the reactor; iii)increasing the temperature of the reactor to synthesis temperature; iv)supplying reactant gases comprising carbon and phosphorus sources intothe reactor and allowing the reactant gases to react with the materialor substrate, introduced into the reactor in step i); v) controlling anyone or more of the temperature of the reaction between the reactantgases and the substrate, the reaction time, and the concentration of thecarbon or phosphorus-containing reactant gases, thereby controlling theshape of the heterogeneous nanowires being synthesized; and vi) coolingthe reactor to room temperature in a carrier gas atmosphere.

In the step of allowing the reactant gases to react with the substratematerial, the supply rate of the gaseous carbon or phosphorus source canbe controlled, whereby the nanowires can have knot-like portions whichchange the thickness of nanowires along the axial direction of thenanowires. Herein, the supply rate of the carbon and phosphorus sourcescan be controlled with time, thereby controlling the length of theknot-like portions. Alternatively, the number of the knot-like portionscan be controlled by controlling the number of changes in the supplyrate of the carbon and phosphorus sources. In addition, whether thegraphitic shells are formed on the surface of the apatite cores can becontrolled by switching on or off the supply of the carbonsource-containing reactant gas.

In yet another aspect, the present invention provides heterogeneousnanowires synthesized by the above method, which are composed ofgraphitic shells and apatite cores and have a thickness which changes inthe thickness of the nanowires along the axial direction of thenanowires thereof. The heterogeneous nanowires may be used asbiomaterials, nanomaterials, or nano/bio composite materials.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawings will be provided by the Office upon request and paymentof the necessary fee.

The above and other objects, features and advantages of the presentinvention will be more clearly understood from the following detaileddescription taken in conjunction with the accompanying drawing, inwhich:

FIG. 1 is a flowchart showing a method for synthesizing single-crystalnanowires sheathed in graphitic shells according to the presentinvention;

FIG. 2 shows photographs before and after synthesis of the inventivesingle-crystal apatite nanowires sheathed in graphitic shells, grown onglass fiber;

FIG. 3 shows a scanning electron microscopy (SEM) image of the inventivesingle-crystal apatite nanowires sheathed in graphitic shells, grown onglass fiber;

FIG. 4 shows a SEM image of the inventive single-crystal apatitenanowires sheathed in graphitic shells, grown on glass powder;

FIG. 5 shows the results of X-ray diffraction (XRD) of the inventivesingle-crystal nanowires sheathed in graphitic shells;

FIG. 6 shows transmission electron microscopy (TEM) image and selectedarea electron diffraction (SAED) images of the inventive single-crystalnanowires sheathed in graphitic shells;

FIG. 7 shows a TEM image of the inventive single-crystal nanowiressheathed in graphitic shells;

FIG. 8 shows TEM and scanning transmission electron microscopy (STEM)images of the inventive single-crystal nanowires sheathed in graphiticshells;

FIG. 9 shows the results of energy dispersive X-ray spectroscopy (EDX)of the inventive single-crystal nanowires sheathed in graphitic shells;

FIG. 10 shows the results of electron energy loss spectroscopy (EELS)mapping of the inventive single-crystal nanowires sheathed in graphiticshells;

FIG. 11 shows the results of X-ray photoelectron spectroscopy (XPS) ofthe inventive single-crystal nanowires sheathed in graphitic shells;

FIG. 12 shows the results of time-of-flight secondary ion massspectroscopy (TOF-SIMS) of the inventive single-crystal nanowiressheathed in graphitic shells;

FIG. 13 shows the results of Raman spectroscopy of the inventivesingle-crystal nanowires sheathed in graphitic shells;

FIG. 14 shows the results of Fourier transform infrared spectroscopy(FT-IR) of the inventive single-crystal nanowires sheathed in graphiticshells;

FIG. 15 shows the results of observation of the inventive single-crystalnanowires sheathed in graphitic shells as a function of synthesis time;

FIG. 16 shows the results of observation of the inventive single-crystalnanowires sheathed in graphitic shells as a function of synthesistemperature;

FIG. 17 shows the results of observation of the inventive single-crystalnanowires sheathed in graphitic shells according as a function of theconditions of reactant gases;

FIG. 18 shows the results of controlling the shape of the inventivesingle-crystal nanowires sheathed in graphitic shells by changing thesupply rate of reactants once during synthesis of the nanowires;

FIG. 19 shows the results of controlling the shape of the inventivesingle-crystal nanowires sheathed in graphitic shells by changing thesupply rate of reactants twice during synthesis of the nanowires;

FIG. 20 shows the results of controlling the shape of the inventivesingle-crystal nanowires sheathed in graphitic shells by changing thesupply rate of reactants three times during synthesis of the nanowires;

FIG. 21 shows the results of controlling the formation of graphiticshells in the inventive single-crystal nanowires sheathed in graphiticshells;

FIG. 22 shows the results of synthesizing the inventive single-crystalapatite nanowires sheathed in graphitic shells, on various substratescontaining calcium;

FIG. 23 shows the results of synthesizing the inventive single-crystalnanowires sheathed in graphitic shells using a variety of carbonsource-containing reactant gases;

FIG. 24 shows an SEM image of the inventive single-crystal apatitenanowires sheathed in graphitic shells, grown on a strontium-containingsubstrate;

FIG. 25 shows a TEM image of the inventive single-crystal apatitenanowires sheathed in graphitic shells, grown on a strontium-containingsubstrate; and

FIG. 26 shows an EDS of the inventive single-crystal apatite nanowiressheathed in graphitic shells, grown on a strontium-containing substrate.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, heterogeneous nanowires composed of graphitic shells andapatite cores according to the present invention and synthesis methodsthereof will be described in detail with reference to the accompanyingdrawing.

FIG. 1 is a flowchart showing a method for synthesizing single-crystalnanowires sheathed in graphitic shells according to the presentinvention.

The inventive method for synthesizing single-crystal apatite nanowirescomprising an apatite core sheathed in a graphitic shell comprises thesteps of: i) introducing into a reactor either a material containing anelement corresponding to X of X₅(YO₄)₃(Z), which is a chemical formulafor apatite, or a substrate containing the material; ii) maintaining theinside of the reactor in a vacuum and supplying a carrier gas to thereactor; iii) increasing the temperature of the reactor to synthesistemperature; iv) supplying reactant gases containing carbon andphosphorus sources to the reactor and allowing the reactant gases toreact with the material or substrate, introduced into the reactor instep i); and v) cooling the reactor to room temperature in a carrier gasatmosphere.

In the method of the present invention, the element corresponding to Xof the chemical formula of apatite may be Ca, K, Na, Sr, Ba, Mg, Pb, Cbor Zn. For example, if the element is calcium, the material containingthe element is preferably a material which contains calcium or calciumoxide or can induce calcium or calcium. Examples of a biomaterialcontaining calcium include henequen and kenaf, which are woodybiomasses, red algae and brown seaweed. Further, the shape of thesubstrate may be selected from mesh, foams, spheres, fibers, tubes,plates, thin films, powders, and nanoparticles. In one embodiment of thepresent invention, the substrate may be glass fiber or glass powder.

In addition, the substrate is preferably mounted in the reactor using anassistant material, such as a crucible or flat panel made of alumina orquartz which is stable to the reactant gases at a temperature rangingfrom room temperature to 1000° C.

Furthermore, in the step of controlling the internal atmosphere of thereactor before synthesis by a vacuum pump and by supply of a carriergas, the internal pressure of the reactor is reduced to a vacuum of1×10⁻³ Torr by means of a vacuum pump in order to remove the remaininggas from the reactor. Then, the carrier gas is supplied to the reactormaintained in a vacuum. The carrier gas may be any one of argon, heliumand nitrogen.

In addition, in the step of elevating the temperature of the reactor tosynthesis temperature, the temperature of the reactor is preferablycontrolled in the range of 500 to 1000° C. while the carrier gas issupplied thereto. As the temperature of the reactor reaches to a desiredtemperature within the above range, reactant gases containing either acarbon source or a phosphorous source or derivatives thereof aresupplied to the reactor.

Moreover, the carbon source-containing reactant gas that is supplied tothe reactor controlled to the above temperature preferably includes ahydrocarbon gas such as acetylene, ethylene, ethane, propane or methane,and the phosphorus source-containing reactant gas preferably includes aphosphine gas.

Herein, the phosphine gas is one example of a reactant gas that containsphosphorus (P) among P, As, V and S, which are elements corresponding toY in the apatite structure.

Also, the carbon source-containing reactant gas is supplied in order toform graphitic shells.

The reactant gases can react with each other during the synthesisprocess to induce new gaseous carbon-phosphorus organic compounds. Ifthe carbon-phosphorus organic compounds are prepared in liquid form,they are preferably evaporated by heating or atomized by ultrasonicatomization, and then supplied to the reactor.

The supplied gaseous carbon source and phosphorus source can generatevarious gases in the above-specified reaction temperature range. Amongthese gaseous derivatives, the carbon-phosphorus organic compounds playa very important role in the nucleation and oriented growth of apatitecrystals. Typical gaseous carbon-phosphorus organic compounds confirmedin the present invention phosphorine (C₅H₅P) and phosphinoline: (C₉H₇P).

The gaseous carbon-phosphorus organic compounds react with a calciumsource material to form amorphous calcium phosphate nanoparticles whichare then subjected to a nucleation and crystallization process with thepassage of time, thereby forming crystalline apatite.

Herein, if the introduced substrate is a calcium-based material,amorphous calcium phosphate nanoparticles start to be formed on thesurface of the substrate as amorphous calcium phosphate compounds aresupplied. In this process, phosphorus molecules thermally decomposedfrom phosphine at synthesis temperature react with the oxygen of thesubstrate to form phosphates which then react with calcium, therebyforming amorphous calcium phosphate compounds. In addition, because thegaseous carbon-phosphate organic compounds resulting from the reactionbetween the carbon and the phosphorus source-containing reactant gas caninduce the strong bonding between the calcium and the phosphate, theycan also promote the formation of amorphous calcium phosphatenanoparticles. Then, the amorphous calcium phosphate nanoparticlesundergo a nucleation and crystallization process, and the gaseouscarbon-phosphorus organic compounds also play a role in promoting thisnucleation and crystallization process and function to induce theapatite crystal to be oriented in one plane that is the (001) plane.When the phosphorus and calcium-containing gaseous species arecontinuously supplied, one-dimensional apatite nanowires oriented in the(001) plane grow, while graphitic shells are formed around the radialsurface of the apatite nanowires under continuous supply of the carbonsource-containing reactant gas. The graphitic shells formed around theapatite nanowires function to block the phosphorus andcalcium-containing gaseous species from being supplied to the radialsurface of the nanowires, thereby preventing the lateral growth of thenanowires. In addition, the formed graphitic shells function to promotethe growth of the nanowire in the axial direction. As a result, theinventive apatite nanowires sheathed in graphitic shells can be formedby a CVD-based vapor-solid (VS) growth mechanism. In this formationprocess, the gaseous carbon-phosphorus organic compounds can play a veryimportant role in the formation and oriented growth of apatite crystals.

The synthesis of such heterogeneous nanowires comprising graphiticshells and apatite cores is continuously performed during the synthesisprocess. If the conditions of synthesis are controlled, heterogeneousnanowires, which comprise graphitic shells and apatite cores and havethe shapes controlled in the axial direction (growth direction) as shownin FIGS. 15 and 16 below, can be obtained.

Particularly, it was confirmed in the present invention that the shapeof heterogeneous nanowires can be freely controlled by controlling thereaction temperature for synthesis, the reaction time, and theconcentration of the carbon source or phosphorus source in the reactantgases being supplied. The heterogeneous nanowires synthesized in thismanner are composed of graphitic shells and apatite cores and arecharacterized in that knot-like structures are formed.

Herein, the length of the knot-like portions can be controlled bycontrolling the supply rate of the carbon and phosphorus sources withtime. Alternatively, the number of the knot-like portions can becontrolled by controlling the number of changes in the supply rate ofthe carbon and phosphorus sources. In addition, whether the graphiticshells are formed on the surface of the apatite cores can be controlledby switching on or off the supply of the carbon source-containingreactant gas.

After the reactant gases are supplied as described above, the reactiontime for synthesis is controlled. In this step, the reaction time iscontrolled within the range of 30 seconds to 2 hours, and the synthesistime can have a direct influence on the growth of length ofheterogeneous nanowires.

After completion of the synthesis, the reactor is cooled in anatmosphere of a carrier gas alone, and finally, heterogeneous nanowirescomposed of graphitic shells and apatite cores can be obtained.

Meanwhile, the present invention provides single-crystal apatitenanowires sheathed in graphitic shells, synthesized by theabove-described method. In a preferred embodiment, apatite comprises99-100% of the inner cavity of the graphitic shells, the nanowires havea diameter of 5 nm to 20 nm and a length of 100 nm to 5 μm, and thegraphitic shells have a thickness of 0.34-2 nm. The heterogeneousnanowires may be used as biomaterials, nanomaterials, or nano/biocomposite materials.

The inventive method for synthesizing heterogeneous nanowires composedof graphitic shells and apatite cores have a very important significancein that it is very simple, and at the same time, makes it possible toachieve the simultaneous synthesis of graphitic shells and apatitecores, which has been considered difficult in the prior art. Inaddition, the method of the present invention is a novel method which ishighly reproducible and can also be applied to a mass productionprocess.

Hereinafter, examples of heterogeneous nanowires composed of graphiticshells and apatite cores according to the present invention will bedescribed. It is to be understood, however, that these examples are notintended to limit the scope of the present invention and may be modifiedin various forms without departing from the scope of the presentinvention.

Example 1 Results of Observation Before and after Synthesis ofSingle-Crystal Apatite Nanowires Sheathed in Graphitic Shells

FIG. 2 shows photographs before and after synthesis of the inventivesingle-crystal apatite nanowires sheathed in graphitic shells, grown onglass fiber. Herein, the prepared calcium-containing material was glassfiber, and the glass fiber used was dispersed uniformly on a quartzplate to form a thin film. The formed glass fiber thin film was mountedin a reactor, and synthesis was performed using argon gas as a carriergas at 750° C. The synthesis time was 1 hour, and acetylene andphosphine gas were used as reactant gases.

Example 2 SEM Image of Single-Crystal Apatite Nanowires Sheathed inGraphitic Shells, Grown on Glass Fiber

FIG. 3 shows an SEM (scanning electron microscopy) of the inventivesingle-crystal apatite nanowires sheathed in graphitic shells, growth onglass fiber. As can be seen therein, the synthesized heterogeneousnanowires were distributed very uniformly on the glass fiber. Thesenanowires had a length of about 1-2 μm and a diameter of 20 nm or less.The synthesized heterogeneous nanowires contained tooth-to-toothcontacts between the glass fibers, suggesting that the nanowires weregrown very densely.

Example 3 SEM Image of Single-Crystal Apatite Nanowires Sheathed inGraphitic Shells, Grown Glass Powder

FIG. 4 shows an SEM image of the inventive single-crystal apatitenanowires sheathed in graphitic shells, growth on glass powder. As showntherein, the heterogeneous nanowires were also distributed uniformlythroughout the surface of glass powder having a particle size of about 5μm. The size of the produced heterogeneous nanowires appeared to besimilar to that of the nanowires grown on glass fiber, but thesenanowires appeared to grow to a final length of 5 μm. Such resultsconfirmed that the shape of heterogeneous nanowires growing on acalcium-containing material is not significantly influenced by the sizeand shape of the calcium-containing material.

Example 4 Results of XRD of Single-Crystal Apatite Nanowires Sheathed inGraphitic Shells

FIG. 5 shows the results of XRD of the inventive single-crystal apatitenanowires sheathed in graphitic shells. As can be seen therein, in the2θ region between 15° and 35°, amorphous patterns of amorphous glassappear. In this region, the major peaks of apatite are also observed. Ataround 2θ=31.8°, a strong peak corresponding to the (211) plane ofapatite is observed. In addition, the peaks corresponding the (300),(112) and (002) planes are strongly observed at around 2θ=32.9°, 32.2°and 25.9°.

Example 5 TEM and SAED Patterns of Single-Crystal Apatite NanowiresSheathed in Graphitic Shells

FIG. 6 shows the TEM image and SAED patterns of the inventivesingle-crystal apatite nanowires sheathed in graphitic shells. Nanowiresdispersed on the TEM grid as shown in the TEM image (FIG. 6( a)) isshown in FIG. 6( b). Although the nanowires are single crystals, thediffraction patterns in the area in which these nanowires are randomlydispersed appear like polycrystalline structures, and such results areindicated by white circular bands in FIG. 6( b). The distance betweenthe white circular bands was calculated, and as a result, the distancebetween the planes was exactly consistent with the results calculatedfor the XRD diffraction patterns shown in Example 4.

Example 6 TEM Image of Single-Crystal Apatite Nanowires Sheathed inGraphitic Shells

FIG. 7 shows a TEM image of the inventive single-crystal apatitenanowires sheathed in graphitic shells. FIG. 7( a) shows the crystalimage of the produced heterogeneous nanowires. As shown therein, theheterogeneous nanowires have a structure in which the nanowire core issheathed in a shell. The shell was observed to have a very thinthickness of less than 2 nm, and the nanowire corresponding to the innercore had a diameter of about 15 nm and was single-crystalline. It wasobserved that the nanowire corresponding to the inner core was grownperpendicularly to the (002) plane of apatite, and this result isevident from the diffraction pattern of the image shown in FIG. 7( a).

Example 7 TEM and STEM Images of Single-Crystal Apatite NanowiresSheathed in Graphitic Shells

FIG. 8 shows TEM and STEM images of the inventive single-crystal apatitenanowires sheathed in graphitic shells. FIG. 8( a) shows an energyfiltered (EF)-TEM image of the nanowires. As can be seen therein, thenanowire has a diameter of 15 nm or less and is sheathed in a thinshell. FIG. 8( b) shows an STEM image of the nanowires.

Example 8 Results of EDX Analysis of Single-Crystal Apatite NanowiresSheathed in Graphitic Shells

FIG. 9 shows the results of EXD analysis of the inventive single-crystalapatite nanowires sheathed in graphitic shells. As can be seen therein,calcium, phosphorus, oxygen and carbon were detected in the nanowire.Herein, the detected copper is attributable to the TEM grid. Inaddition, calcium, phosphorus and oxygen are the fundamental componentsof apatite, and carbon was detected in the grid and the shell of theheterogeneous nanowires.

Example 9 Results of EELS Analysis of Single-Crystal Apatite NanowiresSheathed in Graphitic Shells

FIG. 10 shows the results of EELS mapping analysis of the inventivesingle-crystal apatite nanowires sheathed in graphitic shells. In theresults of EELS analysis of heterogeneous nanowires as shown in theEF-TEM image of FIG. 10( a), calcium, phosphorus and oxygen weredetected in the inner core, and carbon was detected in the outer shell.Such results are completely consistent with the results of TEM and EDXobserved in the above examples, suggesting that the heterogeneousnanowires synthesized according to the present invention are composed ofgraphitic shells and apatite cores.

Example 10 Results of XPS of Single-Crystal Apatite Nanowires Sheathedin Graphitic Shells

FIG. 11 shows the results of XPS of the inventive single-crystal apatitenanowires sheathed in graphitic shells. As can be seen therein, the C(1S) peak corresponding to carbon is the strongest peak. In addition, Ca(2P), P (2p), O (1s) peaks appear. XPS is a system for analyzing thesurface properties of a material, and the results of this exampleindicate that the major component of the shell of the heterogeneousnanowires is carbon and that the major components of the inner core arecalcium, phosphorus and oxygen. Also, the loss peaks of O (1s) can beobserved in typical calcium phosphate compounds and related materialsand show a tendency similar to that observable in apatite.

Example 11 Results of TOF-SIMPS of Single-Crystal Apatite NanowiresSheathed in Graphitic Shells

FIG. 12 shows the results of TOF-SIMS of the inventive single-crystalapatite nanowires sheathed in graphitic shells. In this Example, thesurface of the sample was treated with bismuth ions for a short time.Measurement results for the positive and negative ions of the samplesurface are shown in FIGS. 12( a) and (b), respectively.

For the positive ions, the results related to Ca⁺, CaO⁺, Ca(OH)⁺ andC_(x)H_(y) were measured. It is believed that calcium cations weredetected in the core of the heterogeneous nanowires and that hydrocarboncations were detected in the graphitic shell. In the sample whosesurface was not treated with ions, the peaks corresponding tohydrocarbon cations were more strongly detected.

For the negative ions, the peaks of PO_(x) ⁻, C_(x)P_(y), C_(z) ⁻, O⁻and OH⁻ were mainly detected. It is believed that phosphate anions(PO_(x) ⁻) and oxygen and hydroxide anions were detected in the apatitecore and that carbon-related anions were detected in the graphiticshell. In the sample whose surface was not treated with ions, the peakscorresponding to hydrocarbon anions were more strongly detected.Particularly, it was observed that, after the surface of theheterogeneous nanowires was treated with ions, the amount of hydroxideanions detected increased.

Example 12 Results of Raman Spectroscopy of Single-Crystal ApatiteNanowires Sheathed in Graphitic Shells

FIG. 13 shows the results of Raman spectroscopy of the inventivesingle-crystal apatite nanowires sheathed in graphitic shells. Theresults in FIG. 13 were measured at 514 nm laser source. The peaksappearing at around 1350 and 1580 cm⁻¹ of the spectrum are the D and Gpeaks of carbon detected in the graphitic shells. The peak measuredbetween 900 and 1000 cm⁻¹ at an energy intensity of 5 mW appears tocorrespond to a phosphate-related band detected in the apatite core.

Example 13 Results of IR Spectroscopy of Single-Crystal ApatiteNanowires Sheathed in Graphitic Shells

FIG. 14 shows the results of FT-IR spectroscopy of the inventivesingle-crystal apatite nanowires sheathed in graphitic shells. Peakswhich are typically observed in apatite appear at around 600, 960, 1030and 1090 cm⁻¹. In addition, an IR peak corresponding to multiwall carbonnanotubes was observed at around 1570 cm⁻¹. Also, at around 3570 cm⁻¹, apeak corresponding to OH was very weakly observed. The results of thisExample together with the results of Raman spectroscopy confirm that theheterogeneous nanowires of the present invention are composed ofgraphitic shells and apatite cores.

Example 14 Results of Observation of Single-Crystal Apatite NanowiresSheathed in Graphitic Shells as a Function of Synthesis Time

FIG. 15 shows the results of observation of the inventive single-crystalapatite nanowires sheathed in graphitic shells as a function ofsynthesis time. FIG. 15( a) shows an SEM image obtained 30 seconds afterthe start of synthesis and indicates nanoparticles of very small size onthe substrate surface. It was observed that these particles weretransformed into one-dimensional nanostructures gradually with thepassage of time and had a nanorod shape after 10 minutes (FIG. 15( b)).60 minutes after the start of synthesis (FIG. 15( c)), the producednanostructures had a complete nanowire shape.

Example 15 Results of Observation of Single-Crystal Apatite NanowiresSheathed in Graphitic Shells as a Function of Synthesis Temperature

FIG. 16 shows the results of observation of the inventive single-crystalapatite nanowires sheathed in graphitic shells as a function ofsynthesis temperature. FIGS. 16( a), (b) and (c) show SEM imagesobtained at synthesis temperatures of 650, 750 and 850° C.,respectively. In the SEM image at 650° C., the nanostructures formed onthe substrate surface appear like short irregular nanorods. At asynthesis temperature of 750° C., it is observed that very thin maturenanowires cover the substrate surface in a very uniform and densemanner. At a synthesis temperature of 850° C., it is observed that thediameter of the formed nanowires significantly increases. In addition,the results of additional TEM analysis indicated that the thickness ofthe graphitic shells rapidly increased with an increase in synthesistemperature and this tendency was more remarkable at synthesistemperatures higher than 750° C.

Example 16 Results of Observation of Single-Crystal Apatite NanowiresSheathed in Graphitic Shells as a Function of the Conditions ofSynthesis Temperature

FIG. 17 shows the results of observation of the inventive single-crystalapatite nanowires sheathed in graphitic shells as a function of theconditions of synthesis gases. FIG. 17( a) shows the results obtainedwhen a carbon source alone was supplied as a reactant gas. As can beseen therein, the substrate surface remained relatively clean. Theresults of analysis for the components of the surface indicated thatcarbon in addition to the components of the substrate was detected.

FIG. 17( b) shows the results obtained when a phosphorus source alonewas supplied as a reactant gas. As can be seen therein, smallprotrusions were formed on the substrate surface. In the results ofcomponent analysis, phosphorus in addition to the components of thesubstrate was detected and little or no carbon was detected. The resultsof additional TEM analysis indicated that the obtained nano-protrusionsmostly showed the properties of amorphous calcium phosphate compounds.

FIG. 17( c) shows the results obtained when a carbon source alone wassupplied to a product obtained by supplying a phosphorous source aloneas a reactant gas. The surface shape of the substrate appears to besimilar to that in FIG. 17( b). In the results of component analysis,phosphorous and carbon components in addition to the components of thesubstrate were detected. However, the structures formed on the surfacestill showed an nano-protrusion shape.

FIG. 17( d) shows the results obtained when a carbon source and aphosphorus source were simultaneously supplied as reactant gases. As canbe seen therein, nanostructures formed on the surface all showed ananowire shape. The results of additional TEM analysis indicated thatthe nanowires formed on the surface all have a single-crystal apatitestructure.

Example 17 Results of One-Stage Shape Control Performed During Synthesisof Single-Crystal Apatite Nanowires Sheathed in Graphitic Shells

FIG. 18 shows the results of one-stage shape control performed duringthe synthesis of the inventive single-crystal apatite nanowires sheathedin graphitic shells. In this Example, the supply of carbon andphosphorous sources as reactant gases was changed once during thesynthesis of heterogeneous nanowires. FIGS. 18( a) and (b) show SEM andTEM images of the products obtained by changing the supply once,respectively. The resulting heterogeneous nanowires all had a structurein which the end of each nanowire has one knot-like portion (headportion) having a diameter larger than that of the stem. The results ofTEM analysis of this structure indicated that the stem and the headportion had the same apatite structure and were grown in the samedirection (FIG. 18( c)).

Example 18 Results of Two-Stage Shape Control Performed During Synthesisof Single-Crystal Apatite Nanowires Sheathed in Graphitic Shells

FIG. 19 shows the results of two-stage shape control performed duringthe synthesis of the inventive single-crystal apatite nanowires sheathedin graphitic shells. In this Example, the supply of carbon andphosphorous sources as reactant gases was changed twice during thesynthesis of heterogeneous nanowires. The resulting heterogeneousnanowires all had two knot-like portions (head portions) formed alongthe growth direction of the nanowires. The diameter of the head portionswas larger than that of the stem as in the case of FIG. 18. FIG. 19( a)shows an SEM image of the resulting products. TEM images of the portionsindicated by two arrows in FIG. 19( a) are shown in FIGS. 19( b) and(c). The structure and growth direction of the two formed head portionswere the same as those of the stem of the nanowires.

Example 19 Results of Three-Stage Shape Control Performed DuringSynthesis of Single-Crystal Apatite Nanowires Sheathed in GraphiticShells

FIG. 20 shows the results of three-stage shape control performed duringthe synthesis of the inventive single-crystal apatite nanowires sheathedin graphitic shells. In this Example, the supply of carbon andphosphorous sources as reactant gases was changed three times during thesynthesis of heterogeneous nanowires. The resulting heterogeneousnanowires all had three knot-like portions (head portions) formed alongthe growth direction of the nanowires. The diameter of the head portionswas larger than that of the stem as in the case of FIGS. 17 and 18. TEMimages of the portions indicated by three arrows, in FIG. 20( a)indicated that the structure and growth direction of the formed headswere the same as those of the stem of the nanowires.

Example 20 Results of Control of Formation of Crystalline GraphiticShells During Synthesis of Single-Crystal Apatite Nanowires Sheathed inGraphitic Shells

FIG. 21 shows the results of control of formation of graphitic shellsduring the synthesis of the inventive single-crystal apatite nanowiressheathed in graphitic shells. In FIG. 21( a), a carbon source and aphosphorous source were changed during the synthesis of heterogeneousnanowires such that no graphitic shell was formed on the surface of theresulting heterogeneous nanowires. However, when the sample shown inFIG. 21( a) was reacted with a carbon source, graphitic shells could beformed on the surface of the nanowire as shown in FIG. 21( b). Suchresults suggest that the formation of graphitic sells can be freelycontrolled during formation of the heterogeneous nanowires.

Example 21 Results of Synthesis of Single-Crystal Apatite NanowiresSheathed in Graphitic Shells, on Various Substrates Containing Calcium

FIG. 22 shows the results of synthesis of the inventive single-crystalapatite nanowires sheathed in graphitic shells, on various substratescontaining calcium. In this Example, the following calcium-containingsubstrates were used: FIG. 22( a): an Si—Al—Ca—Na—O-based substrate;FIG. 22( b): an Si—Al—Ca—O-based substrate; FIG. 22( c): anSi—Ca—O-based substrate; FIG. 22( d): a Ca—C—O-based substrate; FIG. 22(e): a Ca—O-HA (hydroxyapatite)-based substrate; and FIG. 22( f): aCa—O-A:C (amorphous carbon)-based substrate. As can be seen in FIG. 22,heterogeneous nanowires composed of crystalline graphitic shells andapatite cores were successfully synthesized using each of the substrate.

Example 22 Results of Synthesis of Single-Crystal Apatite NanowiresSheathed in Graphitic Shells as a Function of the Kind of CarbonSource-Containing Reactant Gas

FIG. 23 shows the results of synthesis of the inventive single-crystalapatite nanowires sheathed in graphitic shells, as a function of thekind of carbon source-containing reactant gas. FIG. 23( a) shows theresults obtained when methane (CH₄) was used as a carbon source. As canbe seen therein, nanostructures are observed on the surface of glassfiber. However, the nanostructures were amorphous in nature and did notgrow into nanowires. FIG. 23( b) shows the results obtained whenethylene (C₂H₄) was used as a carbon source. As can be seen therein,nanoparticles of uniform size are observed. However, these nanoparticlesdid not grow into nanowires. FIG. 23( c) shows the results obtained whenpropane (C₃H₈) was used as a carbon source. Some nanorod-typenanostructures appear in FIG. 23( c). Finally, FIG. 23( d) show theresults obtained when acetylene (C₂H₂) was used as a carbon source. Ascan be seen therein, the nanostructures mostly grew into nanowires. Suchresults can be based on the results of gas analysis obtained during thesynthesis process, and it could be seen that the amount of thecarbon-phosphorous organic compound, phosphorine, in the reactants gaswas higher in the order of FIGS. (d)>(c)>(b)>(a). This demonstrates thatphosphorine plays a very important role in the oriented growth ofapatite nanowires.

Example 23 SEM Image of Single-Crystal Apatite Nanowires Sheathed inGraphitic Shells on Strontium-Containing Substrate

FIG. 24 shows an SEM of the inventive single-crystal apatite nanowiressheathed in graphitic shells on a strontium-containing substrate. It wasobserved that the synthesized heterogeneous nanowires were distributedvery uniformly on the strontium-containing glass powder. Theheterogeneous nanowires appear to have a length of about 2 μm and adiameter of 50 nm or less. It was observed that the synthesizedheterogeneous nanowires grew vertically on the surface of thestrontium-containing glass powder.

Example 24 TEM Image of Single-Crystal Apatite Nanowires Sheathed inGraphitic Shells on Strontium-Containing Substrate

FIG. 25 shows a TEM image of the inventive single-crystal apatitenanowires sheathed in graphitic shells on a strontium-containingsubstrate. It was observed that the heterogeneous nanowires werecomposed of inner nanowires sheathed in shells. It appears that theshells have a very small thickness of about 3 nm and the inner nanowirecores have a diameter of about 20 nm and are single-crystalline innature. It was seen that the inner nanowire cores grew perpendicularlyto the (002) plane of apatite. This result was evident from theadditional analysis of diffraction pattern of the image. As shown in thehigh-magnification TEM image in FIG. 25( a), the material correspondingto the core was clearly single-crystalline in nature, and the directionof growth of the core was perpendicular to the (002) plane. Thedetermined distance between the (002) planes was 0.36 nm, which is wellconsistent with that of the (002) plane of strontium apatite. The FFTdiffraction pattern of the image is shown in FIG. 25( b). The (002)plane and the (100) plane perpendicular thereto are observed in FIG. 25(b). This is well consistent with the hexagonal structure of strontiumapatite.

Example 25 Results of EDX Analysis of Single-Crystal Apatite NanowiresSheathed in Graphitic Shells on Strontium-Containing Substrate

FIG. 26 shows the results of EDX (scanning electron microscopy) of theinventive single-crystal apatite nanowires sheathed in graphitic shellson a strontium-containing substrate. As can be seen therein, strontium,phosphorus, oxygen and carbon were detected. Strontium, phosphorus andoxygen are the fundamental components of strontium apatite, and it wasdetermined that carbon was detected in the amorphous carbon of the TEMgrid and the crystalline carbon shell of the heterogeneous nanowires.Like the results of EDX of the synthesized nanowires, the results of EDXmapping revealed that strontium, phosphorus, oxygen and carbon which arethe major components of strontium apatite were detected. In addition, itwas shown that the ratio of the components was constant throughout thenanowires. This clearly demonstrates that the synthesized nanowires arecomposed of strontium cores and carbon shells.

As described above, the present invention provides the method ofsynthesizing heterogeneous nanowires composed of graphitic shells andapatite cores by introducing into a reactor either a material includingan element corresponding to X in X₅(YO₄)₃(Z), which is a chemicalformula for apatite, or a substrate containing the material,continuously supplying carbon source- and phosphorous source-containingreactant gases into the reactor, and allowing the supplied reactantgases to react with the substrate. In addition, the method of thepresent invention has a very important significance in that it is verysimple, and at the same time, makes it possible to achieve thesimultaneous synthesis of graphitic shells and apatite cores, which hasbeen considered difficult in the prior art, and it can synthesizegraphitic shells and apatite cores in large amounts.

Graphitic shells have very versatile and excellent physical, mechanical,chemical and electrical properties, and apatite shows insufficientphysical and mechanical properties, but has a very highbiocompatibility. Thus, according to the core-shell structure of thepresent invention, the shortcomings of apatite can be significantlyovercome by aid of graphitic sells while the excellent biocompatibleproperties thereof are maintained.

Such results suggest that the inventive heterogeneous nanowires composedof graphitic shells and apatite cores can be used as novel materialshaving excellent physical properties, in the nanotechnology field, thebiotechnology field, and various-nano/bio-technology fields. Inaddition, the material and synthesis technology of the present inventioncan substitute for existing materials and synthesis technologies and canprovide the opportunity of creating new markets.

Although the preferred embodiments of the present invention have beendescribed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

What is claimed is:
 1. A method for synthesizing single-crystal apatitenanowires sheathed in graphitic shells, the method comprising i)introducing into a reactor either a material including an elementcorresponding to X of X₅(YO₄)₃(Z), which is a chemical formula forapatite, or a substrate containing the material; ii) maintaining theinside of the reactor in a vacuum and supplying a carrier gas to thereactor; iii) increasing the temperature of the reactor to synthesistemperature; iv) supplying reactant gases comprising carbon andphosphorus sources to the reactor and allowing the reactant gases toreact with the material or substrate introduced into the reactor in stepi); and v) cooling the reactor to room temperature in a carrier gasatmosphere.
 2. The method of claim 1, wherein the substrate includes oneor more selected from the group consisting of Ca, K, Na, Sr, Ba, Mg, Pb,Cb, and Zn, or oxides thereof, or materials capable of inducing them. 3.The method of claim 1, wherein the substrate is biomaterial containingcalcium, in which the biomass material is selected from henequen andkenaf, which are woody biomasses, red algae and brown seaweed.
 4. Themethod of claim 2, wherein the substrate has a shape selected from thegroup consisting of foam, mesh, sphere, fiber, tube, plate, thin film,powder and nanoparticle shapes.
 5. The method of claim 1, wherein thetemperature of the reactor is controlled to the synthesis temperatureranging from 500° C. to 1000° C. before the reactant gases are supplied,and the reaction time for synthesis is controlled within the range of 30seconds to 2 hours.
 6. The method of claim 1, wherein the carbonsource-containing reactant gas that is supplied to the reactor includesa hydrocarbon gas selected from the group consisting of acetylene,ethylene, ethane, propane and methane, and the phosphorussource-containing reactant gas that is supplied to the reactor includesa phosphine gas.
 7. The method of claim 1, wherein the carbon source-and phosphorus source-containing reactant gases which are supplied tothe reactor induce carbon-phosphorous organic compounds, includingphosphorine and phosphinoline, which play a key role in the nucleationand crystallization of apatite nanoparticles and in the orientation ofthe apatite nanoparticles to the nanowires.
 8. The method of claim 7,wherein, when the carbon-phosphorous organic compounds are in liquidform, they are evaporated by heating or atomized by ultrasonicevaporation, before they are supplied to the reactor.
 9. The method ofclaim 6, wherein the graphitic shells are formed on the radial surfaceof the apatite nanowires under supply of the carbon source, in which theformation of the graphitic shells around the apatite nanowires isperformed by aromatic hydrocarbon molecules produced by cyclization ofhydrocarbons supplied as the carbon source.
 10. A method forsynthesizing single-crystal apatite nanowires sheathed in graphiticshells, which have a lateral change along the axial direction thereof,the method comprising the steps of: i) introducing into a reactor eithera material including an element corresponding to X of X₅(YO₄)₃(Z), whichis a chemical formula for apatite, or a substrate containing thematerial; ii) maintaining the inside of the reactor in a vacuum andsupplying a carrier gas to the reactor; iii) increasing the temperatureof the reactor to synthesis temperature; iv) supplying reactant gasescomprising carbon and phosphorus sources into the reactor and allowingthe reactant gases to react with the material or substrate, introducedinto the reactor in step i); v) controlling any one or more of thetemperature of the reaction between the reactant gases and thesubstrate, the reaction time, and the concentration of the carbon orphosphorus-containing reactant gases, thereby controlling the shape ofthe heterogeneous nanowires being synthesized; and vi) cooling thereactor to room temperature in a carrier gas atmosphere.
 11. The methodof claim 10, wherein, in the step of allowing the reactant gases toreact with the substrate material, the supply of the carbon andphosphorus sources is changed with time, thereby controlling the lengthof knot-like portions which change the thickness of the nanowires alongthe axial direction of the nanowires, and the number of the knot-likeportions is controlled by controlling the number of changes in thesupply of the carbon and phosphorus sources.
 12. The method of claim 10,wherein, in the step of allowing the reactant gases to react with thesubstrate material, whether the graphitic shells are formed on thesurface of the apatite cores is controlled by switching on or off thesupply of the carbon source-containing reactant gas.
 13. Single-crystalapatite nanowires sheathed in graphitic shells, synthesized according toa method as set forth in any one of claims 10 to 12, in which thenanowires have a lateral change along the axial direction thereof.